There are many reasons for patients to incur visual impairment, and in such cases it is important for the ophthalmologist to be able to determine the visual function and predict the visual outcome after treatment. For example, it would be futile for a patient to undergo surgical intervention if the condition of the retina is such that no possibility of improvement exists.
One common cause of visual impairment is macular oedema. Macular oedema results from abnormal accumulation of fluid in the central retina and indicates compromised function in one or both of the blood retinal barriers. It is a common sequel of many ocular conditions and the main cause of visual loss in diabetic retinopathy.
Any abnormal pooling of extracellular fluid may result in displacement of the spatial relationships between retinal neuronal components. Small amounts of fluid may lead to an increase in overall retinal thickness, whilst larger amounts may give rise to cell free spaces as seen in cystoid macular oedema
It is known to predict visual acuity by measuring macular thickness. However, qualitative analysis of data describing the relationship between central macular thickness (CMT) and visual acuity shows that the correlation between CMT and visual acuity is only moderate that CMT is only able to predict 16.6% of visual acuity.